A House-Senate conference on a Senate six-year plan to force a balanced budget opened yesterday in an intense but low-key atmosphere that indicated long, arduous negotiations over implementing the proposal.

Aside from a few biting attacks from adamant House Democratic critics, the session was marked by an absence of the posturing that normally occurs at conferences on highly politicized issues.

Instead, House conferees concentrated on probing the Senate plan for gaps, ambiguities and contradictions ranging from its potential impact in a recession to its effect on relative powers of Congress and the White House.

Near the end of yesterday's session, House Minority Leader Robert H. Michel (R-Ill.) warned against getting bogged down in "endless questions." In response, House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) said the conferees were justified in deeply probing how the Senate measure would work. "No one can convince me that anyone in this room knows exactly what this is about," said Rostenkowski.

The plan calls for annual deficits to fall from $180 billion to zero by 1991 and empowers the president to make across-the-board spending cuts to hit the targets if Congress falls short.

Senate Finance Committee Chairman Bob Packwood (R-Ore.), who is chairing the conference of 48 House members and nine senators, attempted to set the tone by alluding to the plan's bipartisan support in the Senate and the conference's potential for producing "a pinnacle for us and a triumph for the country."

"We've lived too long on borrowed ideas, borrowed money and borrowed time," said Packwood.

Rostenkowski, vice chairman of the conference, outlined House concerns, complaining of a lack of time to consider "a plan that promises to surrender broad powers to the president and threatens the balance of our social responsibility."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) outlined other concerns in a speech to an American Stock Exchange conference here. He said defense spending cuts under the automatic cutback provisions would fall especially hard on spending for readiness, making U.S. defenses "increasingly . . . dependent on inflexible strategic nuclear weapons rather than flexible conventional forces."

He also saw political intent in having heaviest cuts come later. "It allows feeding as usual at the pork barrel next year for the 22 Republican senators up for reelection," O'Neill said, contending that the plan should be called the "Senate Incumbent Protection Act of 1985."

But O'Neill also conceded that some kind of binding budget controls will be enacted. "Do we have to be mandatory about it? The answer is yes," he said.

Concern over time was underscored by testimony from Treasury officials that Congress has only until early next month to pass the debt ceiling extension to which the Senate attached the balanced-budget plan to ensure its passage.

The chief Senate sponsors of the plan -- Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.) -- are not among the conferees. But Gramm and Rudman conducted what amounted to a pep rally for their plan in an adjacent room just as the conference got under way.

The two senators appeared at an informal hearing sponsored by the House Republican Study Committee, whose members heaped praise on them. "People are excited that we are finally going to do something about the deficit," said Rep. Tom DeLay (R-Tex.), adding that it was "amazing and horrendous" that neither Gramm nor Rudman had been named to the conference committee.

The House Republicans also heard an endorsement of the measure by Office of Management and Budget Director James C. Miller III.

Meanwhile, as the Senate prepared to consider a $19 billion segment of a $55.5 billion deficit-reduction package for this year, the OMB complained that the actual savings would be considerably less than the Republican-controlled Senate predicts. Savings over three years would be $65 billion instead of $85 billion, the OMB said.